v4: Type violation upon piping into coerced types #4236

martinjlowm · 2025-04-19T13:29:21Z

edit: supposedly, this is covered in the migration announcement: https://v4.zod.dev/v4/changelog#zcoerce-updates

Testing out v4 I stumbled upon a chain piping into z.coerce.date().

const date = z.union([z.string(), z.number(), z.date()]).nullish().transform((v) => (v === null ? undefined : v)).pipe(z.coerce.date().default(() => new Date()))

Results in,

typescript [2345]: Argument of type 'ZodCoercedDate' is not assignable to parameter of type '$ZodType<any, string>'.
  The types of '_zod.input' are incompatible between these types.
    Type 'unknown' is not assignable to type 'string'.

The intention with this was to accept string | number | Date | null | undefined, map null -> undefined and convert undefined -> Date through the default call such that null and undefined would be converted to the current time.

Seeing this error made me think... and I ended up simplifying it with:

const date = z.null().transform(() => new Date()).or(z.coerce.date())

Handling null first and then afterwards coercing the rest. That being said, v3 typed the pipe correctly without errors, but it may overcomplicate things.

The text was updated successfully, but these errors were encountered:

colinhacks · 2025-05-16T01:08:08Z

I like that schema :)

Zod 4 gets a little more clever with .pipe(). This is why unknown is not always better than any: sometimes the assignability characteristics of unknown causes issues, as in this example.

I used a trick to make the z.coerce APIs a little more flexible in situations like this. Upgrade the latest beta to give it a shot. That said, your code still won't work as is for reasons that are a bit complicated.

You can either, move the default before the pipe (I believe this is functionally identical to your current schema)::

const date = z
  .union([z.string(), z.number(), z.date()])
  .nullish()
  .transform((v) => (v === null ? undefined : v))
  .default(() => new Date())
  .pipe(z.coerce.date());

Alternatively, you can use the generic parameter of z.coerce.date (recently added) to loosen the input type and avoid the assignability issue altogether:

const date = z
  .union([z.string(), z.number(), z.date()])
  .nullish()
  .transform((v) => (v === null ? undefined : v))
  .pipe(z.coerce.date<any>().default(() => new Date()));

4EB3 colinhacks closed this as completed May 16, 2025

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v4: Type violation upon piping into coerced types #4236

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v4: Type violation upon piping into coerced types #4236

v4: Type violation upon piping into coerced types #4236

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